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In a 4-pass firetube boiler, the design typically involves the flue gases traveling through the boiler in a series of passes. The cross-sectional area of each succeeding gas pass is reduced in order to maintain a higher velocity of the flue gases as they move through the system. This increase in gas velocity enhances heat transfer efficiency as the gases make contact with the boiler tubes.

By decreasing the cross-sectional area with each successive pass, the gas flow is accelerated, which helps to improve the overall thermal efficiency of the boiler. A reduced area leads to an increase in the flow velocity, ensuring that the heat from the flue gases transfers effectively to the water within the tubes.

Maintaining an optimal flow velocity is crucial, as too low a velocity can lead to soot formation, and too high a velocity can result in erosion of components. This design consideration is fundamental in ensuring that firetube boilers operate efficiently and safely.